1. Field of the Invention
The present invention relates to an improved heating element which can be used in a process of the type in which a molten material at elevated temperature is poured or dispensed from a container, vessel or the like, as for example a recovery boiler in a wood pulping process, where there is a tendency for the material to freeze or block the outlet of the container, vessel or the like during operation or shutdown of the process. In particular, the invention is directed to a heating element for use in a method of shutting down and starting up, or correcting a freeze up of the process.
2. Description of the Prior Art
Processes are known in which a molten material is discharged from a vessel via some outlet. Illustrative of such processes are those for processing molten metals or metal alloys such as steel, iron, nickel and the like and for processing molten polymers such as polyesters, polycarbonates, polyamides and the like.
Such processes also include the recovery process in pulp mills. Pursuant to present-day pulp mill operations, raw wood is delignified by a thermo-chemical process comprising an approximately 350.degree. F. cook in the presence of sodium hydroxide, sodium carbonate, sodium sulfide and other sodium based compounds. Under such conditions, the lignin binder in the raw wood matrix which holds the natural cellulose fibers together reacts with the sodium and sulfur compounds to form water soluble lignin-sodium complexes thereby permitting a water wash separation of the black tar-like lignin from the fiber for manufacture of paper or other cellulose materials.
Although the sodium compounds used in the afore-described process are relatively inexpensive, the quantities consumed in the production of an average pulp mill necessitate an economical recovery and recycle of the chemical values. Moreover, such sodium-lignin complexes contain sufficient heat value to contribute favorably to the overall mill heat balance. These characteristics are combined in the liquor recovery furnace by fueling a boiler furnace with a concentrated flow stream of the spent or black pulping liquor. Residual ash, predominately sodium carbonate and sodium sulfide falls to the furnace bed as a viscous smelt. Such smelt is removed from the furnace, shattered and dissolved in water to form the green liquor makeup stream from which the other fresh cooking liquor compounds are made.
In transition from the furnace bed to a green liquor dissolving tank, smelt flows in thin continuous streams from numerous spouts around the furnace bed perimeter. Such smelt streams fall directly into the dissolving tank. The smelt typically has a temperature of from about 800.degree. F. to about 1800.degree. F. as it is discharged from the recovery boiler. It is not possible to let this molten stream pour directly into the aqueous solution of the dissolving tank as this would cause a violent explosive reaction. To prevent or minimize violent reaction as the smelt combines with the aqueous green liquor, the smelt spout streams are shattered into small particles as for example by dispersion jets of steam.
A similar problem exists in start-up of a shutdown recovery boiler and during process upset when the smelt in the smelt spout is accidentally or deliberately frozen. During shutdown, the temperature of the boiler is below normal operational temperature and as low as ambient temperature which results in the solidification of smelt in the bottom of the recovery boiler and in the recovery boiler smelt spout. On start-up or during process upset means must be provided to melt the frozen smelt in the spout to return it to operation. In the past, three methods have been used, each having disadvantages.
One method is to melt the smelt in the spout through use of a portable gas burner. This method has the disadvantage of a lack of convenience in the need for a gas supply and fuel supply equipment such as a vaporizer. This method is also time consuming in that the gas burner may have to be assembled and conveyed to the boiler for use.
In the other prior art methods, a rod is used. For example, in one method a metal rod is driven into the frozen smelt to unplug the spout. In the other method, a metal rod is placed in the spout prior to shutdown or upset and is withdrawn prior to start-up to provide an opening to the furnace. These methods also have disadvantages. For example, driving the rod into the frozen smelt often causes damage to the boiler and personal injury to the operator, and is time consuming and may require many hours, i.e., 8 hours or more, to open the spout. In the other method, the rod is often fixed in the frozen smelt and cannot be withdrawn. Each of these methods suffer from the added disadvantage of explosive reaction between the molten smelt and the water on start-up or unpluggage of a frozen smelt spout of a furnace in operation. Normally, as the recovery boiler is started up, the smelt in the bottom of the furnace melts sooner than the smelt plugging the smelt pour spout. Similarly, when the spout is unplugged after an upset, the smelt in the bottom of the furnace is in a molten state. In either case, when the spout is finally opened, a heavy flow of smelt into the water may occur with violent and explosive reaction resulting.
This invention obviates many disadvantages of the prior art processes. For example, this invention reduces the likelihood of a heavy flow of smelt into the water causing a violent and explosive reaction. This invention also provides for greater protection against damage to the boiler and increased speed of freeing a frozen spout as compared to the rod and gas burner methods. The present invention also provides for freedom from gas supply and fuel supply equipment difficulties attendant to the use of gas burner method, and is more reliable than the rod method for keeping a spout open through a shutdown for easy and safe start-up.